Found while verifying -DPIP_HAS_THREADS=OFF / -DPIP_HAS_FILESYSTEM=OFF
smoke builds (pre-existing gaps of the branch):
- piinit.cpp: full implementation now requires PIP_HAS_THREADS as well
(the PIInit class is declared in piinit.h only with threads; without
threads PIInit is absent altogether, stub/absent branches unchanged)
- PIWaitEvent: drop the PIP_HAS_THREADS guard - it is a plain
pipe/select/WaitForMultipleObjects wrapper with no thread usage
(master had no guard), restores unistd.h include for PICAN etc.
- PIEthernet: guard server_thread_ member, server_func(), threaded
listen/stop parts and writeThreaded() calls by PIP_HAS_THREADS
- PIPeer: guard class/impl by PIP_HAS_THREADS (uses PIThread/PITimer/
PIDiagnostics), openDevice() /etc/pip.conf read by PIP_HAS_FILESYSTEM
- PIBinaryLog::restart(): guard threaded-read restart by PIP_HAS_THREADS
- PIIODevice::configure(config_file,...), PIConnection::configureFromConfig,
PIConfig include resolution: guard path-based bodies by PIP_HAS_FILESYSTEM
- PISerial::availableDevicesInfo(): guard /dev and /proc/tty scan by
PIP_HAS_FILESYSTEM
- PIHIDevice impl: requires filesystem (sysfs scan) in addition to threads
- PIPluginLoader::pluginsDirectories/findLibrary: guard PIFile/PIDir use
by PIP_HAS_FILESYSTEM (fallback: empty list / path as-is)
File descriptor 0 is stdin (a valid fd). Using 0 as the 'not-open'
sentinel caused isCreate() to return false and destroy() to skip
close() when pipe() happened to allocate fds {0, 1} (e.g. in daemons
with closed stdio). POSIX convention: -1 = invalid fd.