* src/libs/libgroff/quotearg.c: New file, providing proper argument
quoting for MSVC's spawn* and exec* functions.
* src/libs/libgroff/spawnvp.c: New file, providing a wrapper around
spawnvp with proper quoting for MSVC.
* src/libs/libgroff/assert.cpp (program_name),
src/libs/libgroff/new.cpp (program_name): Declare as `extern "C"'.
* src/libs/libgroff/Makefile.sub (OBJS, CSRCS): Updated.
* src/roff/troff/input.cpp (program_name): Declare as `extern "C"'.
* src/include/error.h (program_name): Declare as `extern "C"'.
* src/include/nonposix.h [__MSDOS__ ...]: Handle spawnvp.
* src/preproc/html/pre-html.cpp [__CYGWIN__ ...]: Declare
spawnvp_wrapper.
[MAY_SPAWN_ASYNCHRONOUS_CHILD]: Declare i and j.