add UIO_MAXIOV macro in sys/uio.h

while there's no POSIX namespace provision for UIO_* in uio.h, this
exact macro name is reserved in XBD 2.2.2. apparently some
glibc-centric software expects it to exist, so let's provide it.

fix up lfs64 junk for preadv/pwritev

add preadv/pwritev syscall wrappers

default features: make musl usable without feature test macros

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

add process_vm_readv and process_vm_writev syscall wrappers

based on a patch submitted by Kristian L. <email@thexception.net>

fix all missing instances of __cplusplus checks/extern "C" in headers

patch by Arvid Picciani (aep)

extensive header cleanup for standards conformance & correctness

thanks to Peter Mazinger (psm) for pointing many of these issues out
and submitting a patch on which this commit is loosely based

initial check-in, version 0.5.0