pear: add options for bin_dir, php_bin, and php_dir settings

packages with executables will put their bins in bin_dir, which is
prefix/pear by default. php_bin can be used if the php interpreter is
in a different location on the target system. php_dir can be used for
overriding the directory where php files are put (useful on debian).
pear: remove debug statement
pear: add more unnecessary pear directories to delete
pear: replace pear_path in executable files

some packages put the pear_path value in their executable (PHPUnit for example)
which is the staging path, so it has to be replaced to the installed path, otherwise
the executable won't work
Merge pull request #358 from oker1/pear_enhancements

Pear enhancements