addmntent: Remove unbounded alloca usage from getmntent [BZ#27083]

addmntent: Remove unbounded alloca usage from getmntent [BZ#27083]

The addmntent function replicates elements of struct mnt on stack
using alloca, which is unsafe.  Put characters directly into the
stream, escaping them as they're being written out.

Also add a test to check all escaped characters with addmntent and
getmntent.