Nettle: Improved compatibility with older Nettles.

In older versions of Nettle, the nettle_*_func typedefs were
function pointers, while in more recent versions they are functions.

Also unifies the naming conventions for Pike-specific typedefs by
renaming the crypt_func typedef to pike_nettle_crypt_func.
Runtime: Fixed buffer overrun in set_default_master().

The string "master.pike" is longer than the string "pike"...

Also fixes C99-ism.
Compiler: Improved cycle detection in implements() and is_compatible().

Under some circumstances the new cache entries could be overwritten
before the functions had completed, leading to infinite recursion.

Fixes [InfoKOM 775534].