extcmd incorrectly assumes PIPE_BUF is real pipe capacity

[Zygmunt Krynicki]

The code inside the glibc-based extcmd implementation is based on the assumption that a pipe may hold at most PIPE_BUF bytes. This is not correct as the actual pipe may be much larger (e.g. 1MB vs 4KB). We need to us /proc/sys/fs/max-pipe-size or use fcntl(2) with F_GETPIPE_SZ to really know how much we need to read.

This causes various bad side effects (manifestations) but is always observable as a "truncated" log file.