Easier to translate psql help

Instead of requiring translators to translate the entire SQL command
synopses, change create_help.pl to only require them to translate the
placeholders, and paste those into the synopsis using a printf mechanism.
Make some small updates to the markup to make it easier to parse.

Note: This causes msgmerge of gettext 0.17 to segfault.  You will need
the patch from https://savannah.gnu.org/bugs/?27474 to make it work.
msgmerge usually only runs on babel.postgresql.org, however.