This commit is the result of the squashing of rfc4_dev branch in a single
commit. It implements mostly RFC 4 related work.

* Grid handling:
  - remove obsolete and presumably unfinished implementation of grid catalog functionality
  - all grid functionality is in grids.cpp/.hpp
  - vertical and horizontal grid shift: rework to no longer load whole grid into memory
  - remove hgrids and vgrids member from PJ structure, and store them in hgridshift/vgridshift/deformation structures
  - build systems: add optional libtiff dependency. Must be explicitly disabled if not desired
  - add support for horizontal and vertical grids in GeoTIFF, if libtiff is available
  - add GenericShiftGridSet and GenericShiftGrid classes, relying on TIFF grids, that can be used for generic purpose grid-based adjustment
  - add a +proj=xyzgridshift method to perform geocentric translation by grid. Used for French NTF to RGF93 transformation using gr3df97a.tif grid
  - deformation: add support for +grids= for GeoTIFF grids
  - horizontal grid shift: fix failures on points slightly outside a subgrid (fixes #209)

* File management:
  - add a filemanager.cpp/.hpp to deal with file related work
  - test for legacy proj_api.h fileapi
  - proj.h: add proj_context_set_fileapi() and proj_context_set_sqlite3_vfs_name() (fixes #866)
  - add capability to read resource files from the user writable directory

* Network access:
  - build systems: add optional curl dependency
  - add a curl-based default implementation for network related functionality
  - proj.h: add C API to control network functionality, and optionaly provide network callbacks
  - add data/proj.ini with default settings
  - add a SQLite3 local cache of downloaded chunks
  - add proj_is_download_needed() and proj_download_file()

* Use Win32 Unicode APIs and expect all strings to be UTF-8 (fixes #1765)
  For backward compatibility, if PROJ_LIB content is found to be not UTF-8 or
  pointing to a non existing directory, then an attempt at interpretating it
  in the ANSI page encoding is done.
  proj_context_set_search_paths() now assumes strings to be in UTF-8, and
  functions returning paths will also return values in UTF-8.

* From openjdk-7 via apt to openjdk-11 via Travis CI xenial's $JAVA_HOME
* From mingw 4.8 -> 5.3, as supported by xenial
* clang+llvm for csa kept at version 6.0.0, but for ubuntu-16.04
* Furthermore xenial implicitly has many upgrades, specifically:
  - gcc 4.8.4 -> 5.4.0
  - clang 4.0.0 -> 5.0.0
  - cmake 3.9.2 -> 3.12.4
  - python 3.4.3 -> 3.5.2
* cppcheck version kept at 1.61 (for trusty) due to many
  false positives with version 1.72 (for xenial)
* Community code contribution documentation simplified to remove
  version of cppcheck, as this may outdated with updates to scripts

* Split global before_install for apt and pip
* Use -qq option for apt-get, which implies -y
* Use list of targets with apt-get rather than one-per-line
* Use local variables for mingw32/install.sh
* Show version info for cppcheck, pip and python
* Simplify clang+llvm-6 path

cmake and autoconf now stipulate C99
change c89 to c99 in travis jobs
remove HAVE_C99_MATH checks

(unrelated) relax Visual Studio compatibility check in
cmake/project-config-version.cmake.in (VS 2019 can use a VS 2015 library
but not vice versa).

If the PROJ_DB_CACHE_DIR environment variable is defined, then a
$(PROJ_DB_CACHED_DIR)/proj.db.sql.md5 file is used to determine if
the set of .sql files has changed since the last time. If not then
$(PROJ_DB_CACHED_DIR)/proj.db is directly used.
This can saved a few seconds when doing rebuilds.
This is a poor man equivalent of ccache for generating the database :-)