make build directory locations less unexpected

currently, qbs has two rather unexpected behaviors relating to build directories:

• as the build dir is named after the config, and the default config is named 'default', one gets a 'default' subdirectory in the source directory when doing an "in-source" build. this is just weird; nobody expects such a name.
• the config name is appended even when doing an explicit shadow build (by specifying -d and/or building from another directory and using -f), so one gets an extra hierarchy level (usually named 'default'), which is also weird.

we can fix this with two modifications:

1. the config name 'default' is never embedded into file names; it's just empty
2. if the build base directory (-d or $PWD) is the source directory, the build directory is prefixed with 'build-' (or just 'build' when rule #1 takes effect)

this yields the following cases:

• default in-source build => src/build
• non-default in-source build => src/build-config
• default shadow build => bld
• non-default shadow build => bld/config

to avoid mistakes, we could complain if -d specifies a parent of the source dir.
it's ok if -d is explicitly set to the source dir itself; rule #2 simply takes effect then.