Cleanup: remove useless makesnmplib variable from build

Squid coding guidelines require automake variables to use naming pattern
ENABLE_* and there is already ENABLE_SNMP defined with intended overlap
of usage. We can drop this unusual variable completely.
Cleanup: remove ClientSocketContextNew() wrapper function

This wrapper function for the ClientSocketContext default constructor
is better performed as an explicit parametered constructor which
prevents accidental use of the default constructor leading to invalid
state after creation (a context always requires connectino and parent
pointers).
Disable error page translation by default in builds

There is no need for every build from VCS to have fresh translations.

Enable explicitly for snapshot and release scripts instead so that
translated template files are still available online and pre-translated
in the distributed packages.
Bug 3927: tests/testRock fatal.cc required
Bug 3996: Malformed DNS reply leads to crash
Fix header-guards in compat/os/hpux.h
Fix header-guard in include/uudecode.h
Fix include guard in include/rfc1123.h
SquidNew: don't redefine new and delete on clang