Add process spawning code from gpgex

* src/common.c, src/common.h (gpgol_spawn_detached): New.
* src/exechelp.h, src/exechelp.c: New.
* src/Makefile.am: Add exechelp files.

While the spawning in gpgol_spawn_detached was quite similar
it makes sense to move that out in it's own source file and
use the same mutex that gpgex uses.