Augment Ubuntu PPA installation instructions.

Ubuntu 10.04 does not recognise `sudo add-apt-repository ppa:blah` out of the box.
README: Remove some information duplicated on mosh.mit.edu
README: Add warning about env var security
README: Mention IRC channel
README: Clarify that mosh-{client,server} don't need to be "installed" per se
AX_CHECK_LIBRARY: Bump serial to match upstream

The m4_ifnblank patch was accepted upstream as
http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commitdiff;h=7ea1d1b18e5c1c3300936dd89c3e55b5b370ae72

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

(closes #153)
Skip 'new' otherwise BSD getopt will think there are no options.

IMO 'new' should be dropped entirely since it's not optional and 'new' is the only choice.

If it were to change in future then it should go after all the options.

(closes #111)
Add ax_check_{compile,link}_flag.m4
Detect binary hardening flags supported by the compiler and linker
Use the hardening flags

Closes #79.
Document the binary hardening measures
Grep for clang warnings when testing compiler flag support

There is no way to make clang's "argument unused" warning fatal.

Ideally the search string would also include 'clang: ' but this output might
depend on clang's argv[0].
Check compiler support for -fno-default-inline and -pipe

clang was spewing warnings about the unrecognized -fno-default-inline.  (Oddly,
it warns only with -c, not when compiling directly to an executable.)  For
completeness we also check -pipe, even though clang is OK with that one.

It should be fine to omit either flag.  gcc -fno-default-inline drops the
implicit 'inline' annotation on functions defined inside a class scope, but
'inline' is only a hint anyway.  -fno-default-inline does not change linkage.
-pipe is merely a compile speed optimization.
Try $LD -Wl,-pie if $LD -pie fails

clang on OS X wants the former.

c.f. http://bugs.ruby-lang.org/issues/5697
Include -fPIE when we check for $LD -pie

configure's linker flag check also compiles a C++ program.  g++ -pie informs
both the compiler and linker, but clang++ -Wl,-pie is only a linker flag, and
the linker will complain that the code hasn't been compiled as
position-independent.
Linux: Use our own signalfd wrapper, rather than libstddjb

selfpipe already does a fine job of interfacing to signalfd.  But Debian and
Ubuntu want us to depend on the skalibs-dev package rather than build libstddjb
ourselves.  That would be fine except that skalibs-dev has static libraries
only, and they aren't built with -fPIC.  This interferes with building
mosh-{client,server} as position-independent executables, which is a desirable
security measure.

So we have our own wrapper, which invokes either signalfd or selfpipe.  And we
build it ourselves with our own flags, because it's part of the Mosh project
proper.

(closes #108)
Close server-related file descriptors before executing child process

Closes #170 github issue.
Handle crazy cursor more gracefully (closes #168)
Fix vttest "Funny scrolling regions" by ignoring some invalid scroll regions

Addresses #155 github issue.
Track icon name and window title separately. Implement MOSH_TITLE_NOPREFIX