farsi.c: Misc. fixes

- Converted some functions to return bools, and changed their respective
  comments to reflect that.
- Minor fixes to a few comments
arabic.c: Comment fixes

- Use C99 style comments everywhere
- Fix incorrect references to return values
coverity/13765,13766,13767,13768: Fix memory leaks in hardcopy.c

The function mch_print_begin() returns early in case of an
error, but without freeing allocated memory.
To fix this, use stack allocation instead of heap allocation
for the variables res_prolog, res_encoding, res_cidfont and
res_cmap.
Travis: Remove Coverity build.

Coverity is now handled by neovim/bot-ci.
Merge pull request #1479 from Pyrohh/fix_comments_and_use_stdbool

{farsi.c,arabic.c}: Fix comments and use stdbool
Merge pull request #1483 from oni-link/fix.mch_print_begin.memory.leak.stackversion

coverity/13765,13766,13767,13768: Fix memory leaks in hardcopy.c
Merge pull request #1157 from fwalch/remove-coverity

Travis: Remove Coverity build.