string_ext: make escape_pattern compatible with Lua 5.2.

Lua 5.2 complains of an illegal character if a non-pattern meta
character is preceded by a '%'.
* std/string_ext.lua (escape_pattern): Escape only meta-chars.
* specs/string_ext_spec.yaml (escape_pattern): Adjust example
code to match.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>