implement hcreate_r, hdestroy_r and hsearch_r

the size and alignment of struct hsearch_data are matched to the glibc
definition for binary compatibility. the members of the structure do
not match, which should not be a problem as long as applications
correctly treat the structure as opaque.

unlike the glibc implementation, this version of hcreate_r does not
require the caller to zero-fill the structure before use.

POSIX conformance fix: define struct entry in search.h

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.

use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

search: add tdestroy (gnu extension)

gnu search.h has struct qelem...

XSI search.h API implementation by Szabolcs Nagy

initial check-in, version 0.5.0