Close #135: adopt full PSR-2 compliance.

With two exceptions this is supposed to be a purely stylistic change
with no functional implications. It achieves full PSR-2 compliance as
reported by PHP_CodeSniffer.

The choice of PSR-2 is mainly one of practicality. It is mostly a
modern standard and by virtue of being PSR it is well known, and it is
used by several mainstream frameworks.

On Debian systems,

    sudo apt-get install php-pear && sudo pear install PHP_CodeSniffer

will install the phpcs binary along with a handful of standards,
including PSR-2. It can then be invoked to print to stdout with

    phpcs --standard=PSR2 sh

PHP_CodeSniffer's default standard is PEAR. You can change this with

    sudo phpcs --config-set default_standard PSR2

and henceforth simply call

    phpcs sh

Unfortunately PHP_CodeSniffer does not support project specific
configuration settings, so it isn't possible to distribute these.

The two exceptions are warnings concerning mixing symbol definition and
logic. One file defined a runtime constant, another a convenience
function. Because of linux-dash's current architecture these definitions
should have had no impact on program behaviour outside their respective
files and so could be safely left in. I opted to resolve the warnings
anyway because

    1) the warnings are good advice;
    2) there would be virtually no runtime or maintenance cost;
    3) it achieves full PSR-2 compliance to the extent we can verify;
    4) it makes invoking phpcs as simple as possible; and
    5) there are no side effects from skipping the relevant sniffs.