scanner: error when element names will not compile

This change checks that the "name" fields of the various structures in
a Wayland protocol XML file will be converted into C identifiers that
can be successfully compiled.

For names which will be inserted as the prefix of an identifier
enforce a match with [_a-zA-Z][_0-9a-zA-Z]* . For types only inserted
as the suffix of an identifier (enum, entry), enforce a format of
[_0-9a-zA-Z]+ .

Unicode characters (and escape sequences like \u0394) are not allowed,
because most older and some newer C compilers do not support them by
default.

For sake of simplicity, this patch does not check for collisions
with reserved words or standard library names.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>