In gutils/gimagewritexbm.c, explain pixel-type error to stderr
In gutils/gimagewritexbm.c, "Can't open file" sent to stderr
In gutils/gimagewritexbm.c, now segfault safe by limiting stem to 255 chars
In gutils/gimagewritexbm.c, allow for possibe internalized period in stem

Don't stop at first period found since it might not be where suffix starts.
In gutils/gimagewritexbm.c, 0==good, anything else can be bad

true/false doesn't allow for other error codes. Use the same logic as fget
and ferror where good==0 and error usually returns a less-than-zero value.
In gutils/gimagewritexbm.c, format output to look nicely aligned.
In gutils/gimagewritegimage.c, add similar updates done in gimagewritexbm.c

Return error message for failed file open. good==0 instead of true.
Verify stem is limited to 255chars. Allow stem to have internal period, so
therefore, look for last period and then truncate suffix.
In gutils/gimagewritegimage.c, format output files with right int types

Ensure we have 32bit where needed, and make alignment look nice on output.
In gutils/gimagewritegimage.c, don't segfault if user sends nothing to do.
In gutils/gimagewritegxpm.c, copied some improvements done earlier

Include added comments. Return error if can't open file. Make sure stem is
255chars or less to prevent seg-fault. Return 0 if good, else !=0 if error.