Use string instead of float as clock device output format

[vinc]

Until now reading /dev/clk/boot or /dev/clk/epoch would return 8 bytes containing a 64 bit float which was good in term of compactness but we might decide to use a different internal representation for those clocks like a 64 bit integer for the number of seconds and a 32 or 64 bit integer for the nanoseconds part.

It also introduced a special case for reading those device files. A better interface would be to use a plain text ASCII string instead, this way we can read those files like any other files. The bytes returned will need to be parsed from string to float but there was already a conversion needed from bytes to float.

It is another breaking change for the next release necessitating to delete the device files and the lisp library and install the new version but there was already a breaking change concerning them:

> delete /dev/clk/*

> delete /lib/lisp/file.lsp

> install