PHP: optimization to avoid surplus chdir(2) calls.

For each request, the worker calls the php_execute_script function
from libphp that changes to the script directory before doing its
work and then restores the process directory before returning.  The
chdir(2) calls it performs are unnecessary in Unit design.  In simple
benchmarks, profiling shows that the chdir syscall code path (syscall,
FS walk, etc.) is where the CPU spends most of its time.

PHP SAPI semantics requires the script to be run from the script
directory.  In Unit's PHP implementation, we have two use cases:

- script
- arbitrary path

The "script" configuration doesn't have much need for a working
directory change: it can be changed once at module initialization.
The module needs to chdir again only if the user's PHP script also
calls chdir to switch to another directory during execution.

If "script" is not used in Unit configuration, we must ensure the
script is run from its directory (thus calling chdir before exec),
but there's no need to restore the working directory later.

Our implementation disables mandatory chdir calls with the SAPI
option SAPI_OPTION_NO_CHDIR, instead calling chdir only when needed.

To detect the user's calls to chdir, a simple "unit" extension is
added that hooks the built-in chdir() PHP call.

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