In gutils/gimagereadxpm.c, change ' ' & '.' to another set of keys

Converted ' ' to '.' and changed '.' to '+' so that the data is visible,
plus if someone happens to save this file with 'delete trailing spaces'
turned-on, then it won't accidently delete the info shown above.

Reworded comment. Missed including free(line) when cleaning-up before exit.
In gutils/gimagewritexpm.c, write mono bitmaps too.
Merge pull request #543 from JoesCat/master

gutils/gimagewritexpm.c, write mono bitmaps too.