Add note on Linux abstract namespace to ipc documentation.
Fix detection of abstract ipc pathname and length calculation.

Abstract socket pathnames must have a NULL character in the first
position, but the second character must also be checked to differentiate
an abstract name from the empty string.  The address length must also
indicate the length of the pathname because the kernel uses the entire
address as the name, including NULL characters.  ZMQ uses
NULL-terminated strings for the address, so the abstract address length
is the length of the string following the initial NULL byte plus 3; two
bytes for the address family and one for the initial NULL character.
Add tests/test_abstract_ipc to .gitignore.