Fix indentation bug when using implements/extends

Correct indentation and highlighting of lines below a <?php is tricky
because that construct is inherently not c-like syntax. Summary of the
solution applied in this php-mode:

* recognize opening tags as preprocessor macros using c-opt-cpp-*,
  this fixes indentation of lines below

* use additional regexp in f-l-keywords to also highlight possible
  closing tag (common in PEAR style code)

* use regexp to work around highlighting glitch, instead of using
  c-type-prefix-kwds because that opens up range of other problems

Added more inline documentation about the custom font-lock expressions
and the handling of open tags.
Prevent variable-face for method invocations

Remove support for php-function-call-face because implementing it
correctly involves more than only fixing it for method invocations.