Committed minor bug fixes and improvements to mhttpd:
1) when generating history plots, use type "double" instead of "float" because "float" does not have enough
significant digits to plot values of large integer numbers. For example, serial numbers of T2K FGD FEB
cards are large integers, i.e. 99000001, 99000002, etc, but when we plot them with offset "-99000000",
the plots show "0" for all cards because when these numbers are converted to "float", they are truncated to
about 5 digits and the least significant digit (the only one of interest, the "1", "2", etc) is lost. Switching to
type "double" makes the plots come out with correct values.
2) fixed breakage of "/History/URL" ODB setting used to offload generation of history plots to a separate
mhttpd process, greatly improving responsiveness of the main mhttpd.
svn revisions 4415-4417