unix: don't use _POSIX_PATH_MAX
unix: don't use _POSIX_PATH_MAX

Libuv was using _POSIX_PATH_MAX wrong. Bug introduced in commit b56d279b
("unix: do not require PATH_MAX to be defined") from September 2018.

_POSIX_PATH_MAX is the minimum max path size guaranteed by POSIX, not
the actual max path size of the system libuv runs on. _POSIX_PATH_MAX
is always 256, the real max is often much bigger.

This commit fixes buffer overruns when processing very long paths in
uv_fs_readlink() and uv_fs_realpath() because libuv was not allocating
enough memory to store the result.

Fixes: https://github.com/libuv/libuv/issues/2965
PR-URL: https://github.com/libuv/libuv/pull/2966
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Jameson Nash <99e7f19e9f64df56d4e81dc00b8566e722cdd766@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso