More robust is_email() and sanitize_email() functions

[sambauers]

Attached patch includes robust and filterable is_email() and sanitize_email() functions.

These functions split up the validation into distinct stages rather than trying to use a single PCRE function. The reason for failure is made available via the filters.

Also included in is_email() is an optional DNS check (where supported by PHP).

Although not quite RFC compliant, it addresses most of the issues in #4616, and also allows easier filtering for those who want to implement RFC compliance via plugins.

[ryan]

(In [10769]) Improved is_email() and sanitize_email(). Props sambauers. fixes #9316 #4616

[azaozz]

(In [10777]) Fix regex function in sanitize_email(), see #9316

[sambauers]

Patch to fix the fix!

[azaozz]

(In [10783]) Fix regex in sanitize_email(), props sambauers, see #9316