Instead of querying sysconf once per loop for a value, we should just
store it during initialization, since it's environment information whose
value won't change during runtime.

Even so, since most of the time we are on musl, this function always
returns -1 and we end up using the BUF_LEN_DEFAULT buffer size.

For now, actually using these buffer lengths isn't implemented, because
the code that uses them is being refactored.

Move BUF_LEN_DEFAULT's value to the one used by glibc in their headers.

musl (as of version 1.2.2) defines NGROUPS_MAX to 32, but glibc and the
kernel define it to 65536. Since musl might change the definition at
some point, and the nss_initgroups interface allows the callee to
realloc the buffer, we don't have to allocate the maximum value upfront,
and can keep using 32 as a reasonable initial default.