Documentation improvements for 2.1.0 (changelog, altcp)

Documentation improvements for 2.1.0 (mainly altcp/altcp_tls)

altcp: default shutdown for both sides is close

doc: altcp: keep altcp_alloc/altcp_free out of the docs

Those functions are for implementing altcp layers, not for applications

Improve altcp allocator documentation

altcp: simplify creating different types by adding an allocator concept

This is done with an example in the http_client

Signed-off-by: goldsimon <goldsimon@gmx.de>

altcp: fix altcp_free

call dealloc function in altcp_free() so it can be propagate it to inner_conn.

altcp: allocate altcp_pcbs from a pool, not from heap (new option MEMP_NUM_ALTCP_PCB defaults to MEMP_NUM_TCP_PCB)

altcp: added altcp_get_port()

Fix function name in altcp.c

httpd/altcp: add forgotten functions

altcp_get_ip: added altcp_get_ip(), added default functions that only defer a call to the inner_conn (moved from tls_mbedtls to generic altcp)

Add altcp to doxygen docs

altcp: added 'addrinfo' and 'tcp_state' functions

Add "application layered TCP connection API (altcp)"

altcp is an abstraction layer that prevents applications linking against the
tcp.h functions but provides the same functionality. It is used to e.g. add
SSL/TLS or proxy-connect support to an application written for the tcp callback
API without that application knowing the protocol details.
Applications written against the altcp API are directly linked against the
tcp callback API for LWIP_ALTCP==0, but then cannot use layered protocols.

Revert "httpd: regenerate example fs (fsdata.c) with current makefsdata"

This reverts commit 66fb52ff5cdfafecef9c0cc7b32bccee8b0f3e7f.

httpd: regenerate example fs (fsdata.c) with current makefsdata