Remove unnecessary #ifdef FRONTEND check to choose between strdup and pstrdup.

The libpgcommon patch made that unnecessary, palloc and friends are now
available in frontend programs too, mapped to plain old malloc.

As pointed out by Alvaro Herrera.
SP-GiST support of the range adjacent operator -|-

Alexander Korotkov, reviewed by Jeff Davis.
Forgot catversion bump in the SP-GiST adjacent support patch.