Get ride of the GMT_exit function

[seisman]

Reported in the PyGMT repository: https://github.com/GenericMappingTools/pygmt/issues/450

The GMT_exit function calls system "exit" function, which works for command line, but also exit external programs (e.g., Python) if API->do_not_exit is set to false. Even though API->do_not_exit is set to true, the GMT library still crashes later, since the fatal error is not correctly handled.

[seisman]

Also reported in https://github.com/GenericMappingTools/gmt/issues/3157.

[joa-quim]

When built with add_definitions(/DDO_NOT_EXIT) it never calls exit What we need to have is to NEVER call exit whatever the circumstances are.

[PaulWessel]

I think most of these happen during map_setup where I pass back a bool search variable and was too lazy to change that when a fatal error occurred. I will see how to do this and maybe I can get help from you to extend the solution to other instances. But deep in #3370 but so close to being done.

[PaulWessel]

FYI, here are the number of occurrences of the string GMT_exit in the source codes:

./gmt_api.c:        6
./gmt_fft.c:        2
./gmt_grdio.c:        4
./gmt_init.c:       29
./gmt_io.c:       22
./gmt_map.c:       17
./gmt_memory.c:        3
./gmt_plot.c:       11
./gmt_proj.c:        3
./gmt_regexp.c:        7
./gmt_shore.c:        4
./gmt_sph.c:       14
./gmt_stat.c:        1
./gmt_support.c:       35
./gmtmath.c:        1
./grdblend.c:        4
./img/img2grd.c:        1
./mgd77/mgd77.c:       25
./mgd77/mgd77manage.c:        9
./segy/pssegyz.c:        4
./segy/segy2grd.c:        3
./spectrum1d.c:        1
./sphtriangulate.c:        6
./spotter/grdspotter.c:        1
./spotter/originater.c:        1
./spotter/spotter.c:       14
./x2sys/x2sys.c:       16
./x2sys/x2sys_datalist.c:        4

So no trivial, and clearly not mostly a gmt_map.c problem.

[PaulWessel]

After #3389 we will be down to these:

./gmt_init.c:       29
./gmt_io.c:       22
./gmt_map.c:       17
./gmt_plot.c:       11
./gmt_sph.c:       14
./gmt_support.c:       35
./gmt_types.h:        2
./mgd77/mgd77.c:       25
./spotter/spotter.c:       14
./x2sys/x2sys.c:       16

and the PSL_exit is gone via #3390. I think I can get the remaining ones done for 6.1.

[joa-quim]

GMT_exit calls are now restricted to mgd77.c and under the control of add_definitions(/DDO_NOT_EXIT) so I think this can be safely closed. (reopen if not).