Make internal header file names consistent

Libcrypto currently has a mess of *_lcl.h, *_locl.h, and *_local.h names
used for internal headers. Move all these headers we inherited from
OpenSSL to *_l

Make internal header file names consistent

Libcrypto currently has a mess of *_lcl.h, *_locl.h, and *_local.h names
used for internal headers. Move all these headers we inherited from
OpenSSL to *_local.h, reserving the name *_internal.h for our own code.
Similarly, move dtls_locl.h and ssl_locl.h to dtls_local and ssl_local.h.
constant_time_locl.h is moved to constant_time.h since it's special.

Adjust all .c files in libcrypto, libssl and regress.

The diff is mechanical with the exception of tls13_quic.c, where
#include <ssl_locl.h> was fixed manually.

discussed with jsing,
no objection bcook

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Add missing $OpenBSD$ markers.

Make this build cleanly even with UTF8_{putc,getc} moved from the
public header to the internal. Make some variables unsigned to
eliminate some warnings

Enable the 3- and 4-byte sequence tests for UTF8_getc()
Add surrogate and out-of-range tests for UTF8_putc() on the assumption
we'll make it return -2. Maybe.

Add regression test for UTF8_{getc,putc}()