remount-fs: modernize coding style a bit

a) Use _cleanup_ where it makes sense

b) Uniformly use negative errno-style errors internally, convert to
   EXIT_FAILURE/EXIT_SUCCESS only when actually exiting.

c) Use log_oom() where appropriate

d) Fix minor memory leak in hashmap addition error path.

e) Don't pretend we could continue sensibly on OOM or fork() failure

f) Use PR_SET_PDEATHSIG to make sure clients we don't kill on error are
   cleaned up.

g) Make use of STRV_MAKE() where it's pretty to do so.

h) Simplify error paths.