This subsystem ensures that we can always use a certain core set of
functions and types, that are either provided by the OS or by replacement
functions / definitions in this subsystem. The aim is to try to stick
to POSIX functions in here as much as possible. Convenience functions
that are available on no platform at all belong in other subsystems
(such as LIBUTIL).

The following functions are guaranteed:

ftruncate
strlcpy
strlcat
mktime
rename
initgroups
memmove
strdup
setlinebuf
vsyslog
timegm
setenv
unsetenv
strndup
strnlen
waitpid
seteuid
setegid
asprintf
snprintf
vasprintf
vsnprintf
opendir
readdir
telldir
seekdir
clock_gettime
closedir
dlopen
dlclose
dlsym
dlerror
chroot
bzero
strerror
errno
mkdtemp
mkstemp (a secure one!)
pread
pwrite
chown
lchown
readline (the library)
inet_ntoa
inet_ntop
inet_pton
inet_aton
strtoll
strtoull
socketpair
strptime
getaddrinfo
freeaddrinfo
getnameinfo
gai_strerror
getifaddrs
freeifaddrs
utime
utimes
dup2
link
readlink
symlink
realpath
poll
setproctitle
memset_s

Types:
bool
socklen_t
uint{8,16,32,64}_t
int{8,16,32,64}_t
intptr_t
sig_atomic_t
blksize_t
blkcnt_t

Constants:
PATH_NAME_MAX
UINT{16,32,64}_MAX
INT32_MAX
RTLD_LAZY
HOST_NAME_MAX
UINT16_MAX
UINT32_MAX
UINT64_MAX
CHAR_BIT

Macros:
va_copy
__FUNCTION__
__FILE__
__LINE__
__LINESTR__
__location__
__STRING
__STRINGSTRING
MIN
MAX
QSORT_CAST
ZERO_STRUCT
ZERO_STRUCTP
ZERO_STRUCTPN
ZERO_ARRAY
ARRAY_SIZE
PTR_DIFF

Headers:
stdint.h
stdbool.h

Optional C keywords:
volatile

Prerequisites:
memset (for bzero)
syslog (for vsyslog)
mktemp (for mkstemp and mkdtemp)