Default executable name should be based on input filename

[dhasenan]

The default executable name is currently set to "a.out". This is traditional, but it's also cruddy. It should be based on the input filename instead, much like the default object file's name.

[0xEAB]

deadalnix commented on #201:
The output is called a.out by default as all major C compiler are doing. SDC has been trying to keep it consistent so there is no surprise.

Two major D compilers (incl. the reference compiler) match the filename, the third one doesn’t.

• DMD: matches filename
• GDC: a.out
• LDC: matches filename

Not sure what’s the ideal solution here; it’s inconsistent already…