Use ClangFormat for code style

[davidstrauss]

Right now, the APCu code has a mix of tabs/spaces and other style inconsistencies. The best tool for rigorous maintenance of code style seems to be ClangFormat, which is also used by the Datastax's Apache Cassandra extension for PHP.

I started with their style and made a few tweaks:

• SortIncludes: false is necessary to maintain #include "php.h" before lines like #include "main/php_streams.h". Otherwise, the build breaks. It's also possible to solve this with /* clang-format off */ blocks or not grouping the includes that must have explicit ordering, but that can always happen later.
• Other options like UseTab: never and TabWidth: 4 to harmonize the ClangFormat configuration with the VIM one included in files.
• Dropping some options that are either defaults for LLVM style or not applicable to APCu.
• A couple other subjective things.

I don't have strong opinions on any parts of the style (not even spaces vs. tabs!). I'm more interested in having a rigid format than which decisions actually get made. So, I'm happy to reroll with different options.

ClangFormat, unfortunately, has limited options around #if block indentation and macro-heavy code. I don't have strong opinions around this, either.

Some options are:

• Just accept how ClangFormat removes some indentation around such "blocks."
• Apply /* clang-format off */ to regions of code heavily dependent on such formatting. I did this for the apcu_functions block, which had rather strange formatting from ClangFormat.
• Exclude the files with the most reliance on manual formatting, like apc_arginfo.h.