merge: optimise #include usage

 * fix cppcheck warnings
 * compose: fix minor dependency problem
 * build: check for <syscall.h> too
 * fix error checking in parse_keymap()
 * fix checks in menu_search()
 * fix checks in progress_choose_increment()
 * fix prototypes in md5.h
 * iwyu: optimise #include usage
 * iwyu: pragma keep some #includes
 * iwyu: pragma exports