Instead of using /tmp/fish as a temporary directory for this operation,
which could lead to clobbering user files, use mktemp to create an
actual temporary directory.
This change does several things. First, it works around a quirk of the
`xgetttext` command that only recognizes description strings in even
numbered position on the command. Second, it allows descriptions
introduced by the `-d` short flag to be recognized.
More importantly, it normalizes the strings so that `xgettext` correctly
extracts them into the *.po file. Prior to this change many fish script
strings were ignored due to how they were written (e.g., single versus
double quotes).
Fixes#4073
