In that mode:
* All non-comment/decoration lines must start with a valid tag
* Commands split on several lines should be terminated with " \"

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.

Should fix the issue reported in https://lists.osgeo.org/pipermail/proj/2020-January/009188.html
Some extra north-american grids present in data/ can affect the results of
some tests, so create a data/for_tests/ subdirectory in which we copy only
select grids.

For reasons unknown to anyone but the geniuses from Redmond: In
MSWindows, the wildcard pattern "*.gie" expands to all files having
an extension *starting* with ".gie".

In other words "dir *.gie" will list files with extension ".gie"
*and* with extension ".gie_failing".

This means that running "gie test\gigs\*.gie" will run all gigs
tests, even the ones known to fail.

So rather than getting a quick check for regressions, one gets
a lot of noise and no easy way to be sure whether any regressions
have appeared.

To work around this oddity, we rename all files ending in
".gie_failing" to end in ".gie.failing" - and while at it,
adding all the non-failing gigs tests to the AppVeyor test suite.

This extends the regression test suite, run at the end of each
AppVeyor integration, to over 2500 individual tests.

Test coverage, however, is still hovering just below 75%, so
there's still plenty of room and reason for additional tests.