lib: var-expand - Add table size and merge utility functions

lib: API change - var_expand_func_table.func() can now return error. None of the existing functions were changed to return errors (yet).

lib: API change - var_expand*() now returns error string. This allows callers to fail properly if the format string is invalid.

lib: var-expand: Removed var_expand_table_build(). Replaced it with explicit table construction at the one place it was used. Clang -Wvargargs complained about passing a char argument to va_start().

lib: var_has_key() properly ignores key=='\0' now.

lib: var_get_key() didn't handle %{long_variables} correctly This also fixes var_has_key()'s long_key handling.

Marked functions parameters that are allowed to be NULL. Some APIs were also changed. The non-obvious APIs where NULL parameter was changed to "" are master_service_init() and auth_master_user_list_init(). These checks can currently be enabled only on a patched clang: http://llvm.org/bugs/show_bug.cgi?id=6786

liblib: Added var_get_key_range()

liblib: Added var_expand_with_funcs() to expand variables with function callbacks.

var_expand(): Added support for long %{variable} names. --HG-- branch : HEAD

Added var_has_key(). --HG-- branch : HEAD

Added more consts, ATTR_CONSTs and ATTR_PUREs. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added var_expand_table_build() --HG-- branch : HEAD

Auth cache key was built wrong if any modifiers were used for %vars. Added var_get_key() function which skips over any modifiers and returns the actual key. --HG-- branch : HEAD

Fixed var_expand() to take a table of variables rather than a few predefined ones. Added support for modifiers. --HG-- branch : HEAD

Added var_expand() --HG-- branch : HEAD